This application relates to high voltage DMOS devices.
Circuit applications often require high voltage and low voltage components on a monolithic integrated circuit. Often multiple high voltage devices are needed. Such circuits have greatly proliferated into various applications as they make possible energy efficient solutions to lighting, motor drivers and power supplies. A half bridge is formed by a low side and a high side transistor. Due to many advantages, in such applications either DMOS (double-diffused metal-oxide-semiconductor) or, for higher current, IGBT (insulated-gate bipolar transistor) devices are used. There are advantages to having a high side DMOS to form the half bridge configuration. Low side DMOS transistors have been produced in monolithic, junction isolated processes for some time, but high side DMOS transistors have only been produced in the same with very limited breakdown voltage. In many applications designers use a low voltage control circuit and discrete DMOS transistors to build a complete circuit.
Building a high side DMOS is more complicated than building a low side DMOS because the p-body and the source will be at high voltage when the transistor is turned on. In traditional monolithic processes, the epi (epitaxy) is too thin to support high voltage between the source/body layers and the substrate. N-channel DMOS use a p-type body diffusion and it will punch through to the p-type substrate at relatively low voltage. It is possible to build the device on a much thicker epi; however, the low voltage part of the circuit needs an isolation diffusion that penetrates the epi at least half way or more. And a p-type buried layer needs to be used that penetrates the epi at least half way to meet with the isolation diffusion. This is the up-down isolation scheme to minimize the lateral diffusion. Making deep isolation diffusions is possible, but at the expense of having a wide isolation diffusion due to the lateral diffusion of the dopant (such as boron) taking up a large area on the chip.
The usual architecture of these types of circuits is such that they use only a few high voltage transistors and many low voltage transistors, the low voltage transistors forming a complex control circuit. The low voltage components are small but require isolation between them. As such, the deep, and necessarily wide, isolation diffusion can easily double the area of a low voltage device, making it impossible to build an economical monolithic integrated circuit with a thick epi. The cost of a monolithic integrated circuit is proportional to the complexity of the manufacturing process to make it and to the area of the chip. Workers in the field have expanded a major effort to reduce the size of the chip while maintaining its level of performance. The area factor is important in high voltage devices as the depletion layer spreads much more with increasing voltage. Various techniques were introduced to minimize the area the depletion layer needs, such as the introduction of the epi extension diffusion, selective epi growth and refill, or the RESURF (re-entrant surface field) principle. These techniques vary in usefulness in building monolithic integrated circuits where a mixture of low voltage and high voltage components are needed.
In typical prior art arrangements, high voltage half bridge circuits use discrete transistors. Monolithic circuits are limited to about 100V. High voltage DMOS transistors have been made using SOI (silicon on insulator) technology. Due to the complete oxide isolation, the transistors can be used as high side devices and low side devices. Some examples of DMOS device are given in U.S. Pat. Nos. 4,811,075; 5,155,568; 6,236,100; 6,573,550; 6,992,362; and 8,324,684.